Applications for embodiments of the present invention may be found in the general area of luminaires, this may include but is not limited to: uplighters, downlighters, wall-washers and distributed arrays of luminaires. More generally, this system can be employed in any luminaire where a point-like nature of an illumination source may otherwise provide an unwelcome excessive intensity. Such excessive intensity is known to be discomforting.
LEDs are being used increasingly as the light source of choice for many lighting applications. A small physical footprint, high efficiency and longer lifetimes mean that LEDs offer considerable advantages over conventional light sources. As the efficiency has increased the luminous output has increased without significant change in the size of the emitter area. In addition, advances in thermal management techniques and improved substrate materials have meant that it is possible to make bigger devices that can withstand higher current densities. Currently die sizes for the high output devices are known to vary from approximately 350×350 microns to 1×1 mm or more.
Thus, technological advances in manufacturing and packaging have contributed to produce devices that are effectively point sources but which are known to be able to emit, for example, 70-80 lumens from a 5 W input using three discrete emitters within a package area of a few square millimetres. However, such devices emit light over a relatively small surface area, and are therefore often undesirable for use in general luminary applications.
Attempts have been made to overcome this problem particularly with such emitters used as the light source in a packaged luminaire. Indeed many examples of relevant prior art systems can be found in which an LED is embedded into or surrounded by a refractive surface or a reflective surface to increase the surface area of the devices luminance. Many standard LED lenses now exist which combine some element of reflection via total internal reflection at one surface together with a refractive element, for example a Fresnel lens surface. See for example U.S. Pat. No. 5,898,267. These lenses have the advantage of being very efficient and can control the light distribution, sometimes making it collimated and in other examples making it diverge at a particular rate.
Whilst these lenses serve a specific purpose many suffer from the problem that they require appreciable depth to create the extended emitter surface and control the light. Such appreciable depth limits the applications that such a device can be used for and is thus an undesirable feature of such devices. U.S. Pat. No. 6,283,613 discloses such a luminaire device.
In some applications the size of the package sets a limitation on the depth of the optical controller. In extreme examples, a lens or other such optical controller only 2-3 millimetres deep may be required. When such small depths are applied to the above-described luminaire device the LED is so close to the optical controller surfaces and the controller is so thin, that Fresnel facets limit the device's effectiveness. Hence such devices are limited by having a minimum depth. Often the result of this arrangement is that the central intensity of the LED dominates any attempt at optical control. In such circumstances a direct view of the emitter can be seen. This is known to be uncomfortable for observers, akin to glare or hot spots.
Further attempts to convert an LED light source from a point source into an extended light source have been tried. For example, U.S. Pat. No. 6,582,103 discloses a method wherein a reflective cavity in which the LED or point source is situated, is combined with a cuspated optical diverter. Light from the LED is distributed by the optical diverter onto the reflective surfaces of the cavity. Before exiting the cavity light passes through a conditioning element which comprises a sheet diffuser and a prismatic sheet such as brightness enhancing film. Although this technique achieves the desired effect of converting the point source into an extended source it does require sufficient physical size to include a reflector cavity and various optical components. For example, a depth of approximately less than 3.5 inches is known to be required between the source and the sheet that illuminates the reflective surfaces. Hence this type of device solution cannot produce an extended light source within a depth of only a few millimetres. Again this method limits the applications in which such a device can be used in. Hence such devices are known to be undesirable.
Others have attempted to solve this problem by using optical sheets for spreading the light. For example, U.S. Pat. No. 5,668,913 discloses a light expanding system that converts a point light source into a collimated linear or planar output. The device comprises a light source, together with a beam collector and a light pipe adjacent to a multiplicity of prismatic elements. For various reasons this arrangement cannot be placed directly over the source and limited in depth to a few millimetres. Consequently such devices are known to be unsatisfactory.
In U.S. Pat. No. 6,456,437 an optical sheet with a structured surface is disclosed in which surface prisms refract incoming collimated light and other prisms use total internal reflection to reflect the incoming collimated light. By varying the prism design and randomly alternating the prism type, a collimated beam can be spread out in angle-space. Although this design spreads the light out it also requires the light from the point source to be collimated by an intermediate optical component. Even the fastest lens with an f-number of 0.5 would need to be at least half the depth of the extended source length. Hence, this design requires a reasonably large depth. Such depths are known to be undesirable.
Another attempt to solve this problem is disclosed in U.S. Pat. No. 7,072,096 in which the output from LED arrays is concentrated within a limited angular range. The light is controlled by surrounding each LED with a reflecting side-wall that directs light onto a prismatic film. In such a device the reflector walls must be deep enough to control the light and it is therefore not suited to a low profile application.
Other attempts have been made to solve this problem by modifying the external light intensity distribution by using a reflective surface. U.S. Pat. No. 6,674,096 discloses using an encapsulation around the LED in which a depression is made directly over the emitter surface. The depression has a predetermined curvature symmetrical about the optical axis of the LED. The curved surface is then reflectively coated. Light rays emerging from the die are reflected at normal to the optical axis of the LED and are refracted at the encapsulant-air boundary. In this manner the point-like light source is converted into an annular emitter. Although useful in certain applications this invention is only of use in creating a side emitter and not an extended area source. The limited applications of such a device thus make it somewhat undesirable.
EP 1 589 282 A1 discloses a thin plate light for motor vehicles comprising a transparent plate between two reflective surfaces. The primary reflective surface covers the whole of the bottom of the plate area and is designed to reflect light rays out through the front of the transparent plate. A secondary reflector is formed on the front surface of the plate, directly in line with the point source, which is located within the transparent plate and coincident with the bottom surface. The secondary reflector is designed so that its aperture extends across the front of the plate so that a ray striking the front plate directly will always be totally internally reflected back towards the primary reflective surface. The primary reflective surface will reflect this ray so that the ray strikes the front surface a second time but now at normal incidence. The ray thus exits the lamp. The curvature of the secondary reflective surface is designed so that rays are reflected from the surface onto a different portion of the primary reflective surface which has been designed to reflect these rays out through the front face of the plate. Thus each zone or facet of the primary reflective surface has been designed to co-operate either with rays reflected by total internal reflection or from the secondary reflective surface. However, it is an inevitable consequence of this design that light cannot escape from the central region of the lamp, which is covered by the secondary reflector. Such uneven light distribution is known to cause discomfort when viewed directly or due to the uneven light distribution it creates on objects.